Ncert Exercises & Solutions

Diatomic molecules like hydrogen have energies due to both translational as well as rotational motion. From the equation in kinetic theory, $PV = \dfrac{2}{3}E$ $,$ $E$ is:

1.	the total energy per unit volume.
2.	only the translational part of energy because rotational energy is very small compared to translational energy.
3.	only the translational part of the energy because during collisions with the wall, pressure relates to change in linear momentum.
4.	the translational part of the energy because rotational energies of molecules can be of either sign and its average over all the molecules is zero.

(3) Hint: Use the kinetic theory of gases.

Step 1: Find the pressure in terms of energy.

According to kinetic theory, we assume the walls only exert perpendicular forces on molecules. They do not exert any parallel force, hence there will not be any type of rotation present.
The wall produces change only in translational motion.
Hence, in the equation,

PV = \frac{2}{3} E

[where P = pressure]

E is representing only the translational part of energy.

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